Xuan Haijun

Impact response and damage evolution of triaxial braided carbon/epoxy composites. Part I: Ballistic impact testing

To investigate the high-velocity impact response and damage evolution of the triaxial braided composite fan case, a series of ballistic impact tests using blade-like projectiles were conducted. Cylindrical projectiles of the same cross-section perimeter were also employed to identify the influence of projectile geometry. In addition, satin woven composites were tested for comparison with respect to failure modes and damage shape. Experimental results indicate that the main failure modes for the two different fiber reinforcement architectures are similar, that is, fiber shear failure and matrix crush failure in the impact surface and fiber tensile failure, fiber pull-out, matrix cracking, and delamination in the exit surface. The damage area in the exit surface is diamond-shaped for satin woven composites while rounded or elliptic for triaxial braided composites according to projectile geometry. The triaxial braided composites have an improved ballistic resistance and higher ballistic limit than satin woven composites. Based on damage area and failure modes observed from experiments, ballistic behavior was predicted using an analytical model, which proves to be accurate enough.

Impact response and damage evolution of triaxial braided carbon/epoxy composites. Part II: finite element analysis

A series of ballistic tests on triaxial braided carbon/epoxy composites were described in Part I of this paper. In this part, numerical simulations were carried out to investigate the impact response, damage evolution, and penetration mechanisms of these composites. A continuum finite element model was developed to obtain the time history of the projectile velocity, displacement, penetration resistance force, and energy absorption during the impact process. By fitting the numerical data, the ballistic limit velocity can be obtained. Good agreements were achieved between numerical results and experimental results. Numerical predictions for ballistic tests of the composites indicate that, based on variations of damage mechanisms and failure features, the impact process can be subdivided into three stages, i.e. phase I – shock compression, phase II – bulge deformation, and phase III – penetration process, among which phase II consumes most of the projectile kinetic energy. The differences between a blade-like projectile and a cylindrical projectile in the impact process are also addressed in detail.

Dynamic Response of a High Speed Flexible Rotor Due to Sudden Large Unbalance

Structure vibration response of rotor-bearing-case-installation section system caused by sudden unbalance load due to blade out of the high-speed flexible rotor has been the focus in the field of aero-engine development. This paper deals with the sudden unbalance response characteristics of a high-speed flexible rotor during a blade out event. To reveal the unbalance response influence of blade released mass and rotating speed, a flexible rotor-damping support-protection support system is constructed using LS-DYNA. This model consists of rotor, disk, single blade, protection support and damping support. The diameter of the disk is designed to be 196 mm and the mass of a released blade is 33 or 66 g with two different widths. It can be seen from the shearing stress results that the rotor shaft is twisted off at a weak key point. After single blade out, rubbing between rotor and protection support is classified as the following two stages. It is observed that as single blade released out, higher rotating speed and larger unbalance mass lead to more violent vibration and cause more severe impact force transmitted to bearing and installation section. They can also lead to less stay time during the first stage mentioned above. To verify the results of simulation, blade out tests are carried out on a high speed spin tester. Severe vibration happens on the support base in the course of shutting down, but rotor has no apparent damage in single blade out test with small mass and low speed. However single blade out event with larger mass and higher speed can cause more danger. Large and long-term unbalance on protection support-support base-protection ring-hanger makes the system vibrate severely and these can lead to break of the support base and twist of the rotor shaft. It can be concluded that the results of tests turn out to be agreed well with those of simulation analysis.